Chap-3: Power TrainMotor Vehicle Engineering

MEng 5165

By: Shewangizaw W

3. Power Train

3. Power trainIn a vehicle, the mechanism that transmits the

power developed by the engine to the wheels is called the power train.

The power train serves two functions: 1. transmits power from the engine to the drive

wheels2. varies the amount of speed and torque. The two common sets of gears in power train are:1. Transmission gear2. Differential gear

DRIVE ARRENGMENTS: (2WD, 4WD) 2WD (2-Wheel Drive)

Part time 4 Wheel DriveFull time 4 Wheel drive

3.1 CLUTCH

Clutch capacity depends on the following factors:Effective radius of the friction surface Coefficient of friction acting between the friction surfaces No of friction surface Clamping force holding the friction surfacesClutch requirements: It must engage the engine and transmission/transaxle

smoothly, Once it engages, it must transmit power entirely without

slipping, It must disengage the transmission/transaxle

accurately and quickly to permit gear change when a vehicle is in motion.

3.1.1 COMPONENT OF CLUTCHAn engine clutch consists primarily of four parts: 1. Fly wheel 2. Friction/clutch disc 3. Pressure plate assembly 4. Release/control mechanism 3.1.1.1 INSIDE THE CLUTCH HOUSING: The clutch housing is a stamped or cast metal part, which

houses the clutch and connects the transmission housing to the back of the engine.

1. Fly wheel2. Friction/clutch disc3. Pressure plate assembly

3.1.1.2 Pressure plate assembly with coil spring:

3.1.1.3 Pressure plate assembly with Diaphragm type spring:

3.1.3. CLUTCH RELEASE MECHANISM3.1.3.1 Operating mechanisms There are two types of clutch, distinguished by the way they are operated the hydraulically operated clutch and the mechanically operated clutch using a cable linkage.3.1.3.2 Mechanical type clutch

3.1.3.3 Hydraulic clutch

3.2 MANUAL TRANSMISSION/TRANSAXLEIt is an assembly of gears and shafts to transmit the rotation and torque of the engine to the driveline or final drive. The operating principles of the gears, shafts, bearings and shift mechanisms in manual transmissions and transaxles are basically the same.

3.2.1 PURPOSE OF A MANUAL TRANSMISSION: If in proper operating condition, a manual transmission should do the

following • Be able to increase torque going to the drive wheel for quick

acceleration, • Supply different gear ratios to match different engine load conditions, • Have a reverse gear for moving the vehicle backwards, • Provide the operator with an easy means of shifting transmission

gears, • Operate quietly with minimum power loss.

3.2.2 MAJOR COMPONENTS OF A MANUAL TRANSMISSION:

The following are most common components 1. Transmission Case (20)2. Extension Housing (21) 3. Transmission Shafts:a. Input shaft.(1)b. Countershaft.(12)c. Reverse idler shaft.(14)d. Main shaft.(5)4. Transmission Gears :- can be classified into four

groups- input gear, countershaft gears(11), main shaft gears(4,6,7,8), and the reverse idler gear(13).

5. Shift Linkage and Levers:- are the EXTERNAL ROD and the INTERNAL SHIFT RAIL. They both perform the same function. They connect the shift lever with the shift fork mechanism.

3.2.3 GEAR BOX 3.2.3.1 Sliding mesh gear box

3.2.3.2 Constant mesh gear box:-has the following advantagesi) It allows the use of helical gears; ii) It is quieter in operationiii) It is stronger than the spur type as there is more than one tooth in engagement at any one timeiv) It makes gear changing easier

3.2.3.3 Synchromesh gearboxes

3.2.4 Transaxle

• The transaxle is a power transmission device attached to one end of the engine, which combines the function of transmission and the drive axle (final drive and differential) into a single assembly.

• There are two basic types of transaxles: Manual and Automatic. Manual transaxles are shifted manually, or by hand. Automatic transaxles are shifted automatically, with no help from the driver.

3.3 Transfer Case :- is an auxiliary transmission mounted in back of the main transmission. By shifting gears in the transfer case, engine power is divided and transferred to both the front and rear differentials.

3.3.1 Types of transfer cases : have been used with a variety of manual and automatic transmissions there are two general types:

1. Part time transfer case : unit also has a gear shift that sends power to only the rear wheels, or to both the front and rear wheels.

2. Full time transfer case : In this type power is available to all four wheels at any time.

In most vehicles, a transfer case provides the driver with a selection of either of two drive speeds (ranges), high or low. High speed in the transfer case provides direct, or a gear ratio of 1:1. Low speed usually provides a gear ratio of about 2:1.

3.4 AUTOMATIC TRANSMISSION It is designed to match the load requirements of the vehicle to the power and speed range of the engine. It performs its function depending on:1. Throttle position 2. Vehicle speed 3. position of the shift lever3.4.1 Advantage of AT over MT Interruption free acceleration It reduces driver fatigue It automatically and smoothly shifts gears at speeds appropriate to the

driving conditions It prevents the engine and drive line from becoming overloaded3. 4. 2 M AJOR COM PONENTS AT Torque converter Planetary gear unit Hydraulic control unit Manual linkage Automatic transmission fluid

3 . 4 .2 . 1 TORQUE CONVERTER:- is mounted on the input side of the transmission gear train and is bolted to the rear end of the engine crankshaft via the drive plate.

3. 4. 2. 1.1 Roles of torque converter

Multiplying the torque generated by the engine. Serving as an automatic clutch Absorbing the torsional vibration of the engine and drive train. Serving as a flywheel to smooth out engine rotation. Driving the oil pump of the hydraulic control system. 3. 4. 2.2. PLANETARY GEAR UNIT:-is housed in a transmission case

made of aluminum alloy. It changes the transmission output rpm and/or the direction of t he output rotation, and transmits it to the final drive unit.

3. 4. 2.2.1 Roles of planetary gear unit Providing several gear ratios to obtain proper torque and

rotational speed in accordance with the driving conditions and driver's desires.

Providing reverse gear for reverse traveling. Providing a neutral gear position to allow the engine to idle while the

vehicle is stopped.

3. 4. 2.2.2 PLANETARY GEAR SET

3. 4. 2. 3. HYDRAULIC CONTROL SYSTEM The hydraulic control system consists of the oil pan, which acts as the fluid reservoir; the oil pump, which generates the hydraulic pressure; various valves having various functions; and fluid passages and tubes, which deliver the transmission fluid to the clutches, brakes, and other components of the hydraulic control l system.

3. 4. 2. 3.1 Roles of hydraulic control system Supplying transmission fluid to the torque converter. Regulating the hydraulic pressure generated by the oil pump. Converting the engine load and vehicle speed into hydraulic

"signals". Applying hydraulic pressure to the clutches and brakes to

control planetary gear operation. Lubricating rotating parts with fluid. Cooling the torque converter and transmission with fluid.

3. 4. 2. 4. MANUAL LINKAGE :the automatic transmission up-shifts and down-shifts automatically. However, two linkages allowing manual operation by the driver are connected to the automatic transmission. These linkages are the selector lever and cable, and the accelerator pedal and throttle cable.

3.4.2.4.1 SHIFT SELECTOR LEVER (SHIFT LEVER)

3. 4. 2. 5 AUTOMATIC TRANSMISSION FLUID (ATF)A special high-grade petroleum-based mineral oil

mixed with several special additives is used to lubricate the automatic transmission. This oil is called automatic transmission fluid (abbreviated "ATF") to discriminate it from other types of oil

3. 4. 2. 5.1 Roles of ATF Transmission of torque in the torque converter.Control of the hydraulic control system, as well as of

clutch and brake operation in the transmission section. Lubrication of the planetary gears and other moving

parts. Cooling of moving parts.

3.4.3 COMPONENT AND PRINCIPLE OF OPERATION OF TORQUE CONVERTOR

1. Converter case- normally made of two pieces of steel welded together in a doughnut shape, housing the impeller, stator, and turbine. The housing is filled with transmission fluid.

2. Pump impeller- driving member that produces oil movement inside the converter whenever the engine is running. The impeller is also called the converter pump.

3. Turbine runner- a driven fan splined to the input shaft of the automatic transmission. Placed in front of the stator and impeller in the housing. The turbine is not fastened to the impeller but is free to turn independently. Oil is the only connection between the two.

4. Stator- designed to improve oil circulation inside the torque converter. Increases efficiency and torque by causing the oil to swirl around the inside of the housing.

3.4.3.1 PRINCIPLE OF POWER TRANSMISSION: The torque converter works in the same way as the two fans below. Fan A is switched ON and fan B is OFF. The pump impeller plays the part of fan A and the turbine runner plays the part of fan B. the medium in the torque converter is the transmission fluid rather than air.

3.4.3.2 PRINCIPLE OF TORQUE MULTIPLICATION If a duct is added, the air will pass through fan B (driven fan) and back

through fan A (drive fan) from behind via the duct, as shown above. The energy remaining in the air after passing through fan B will assist the rotation of the vanes of fan A.

3.4.3.3 OPERATIONS OF TOEQUE CONVERTOR: The primary action of the torque converter results from the action of

the impeller passing oil at an angle into the blades of the turbine. The oil pushes against the faces of the turbine vanes, causing the turbine to rotate in the same direction as the impeller.

The basic operations are:A. Idling: With the engine idling, the impeller spins slowly.B. Acceleration: During acceleration, the engine crankshaft, the

converter housing, and the impeller begin to move faster.C. Torque multiplication: occurs when the impeller is spinning faster

than the turbine.

STATOR

It is located between the pump and the turbine and is mounted on a one-way clutch that allows it to rotate clockwise but not counterclockwise.

Stator action is only needed when the impeller and turbine are turning at different speeds.

The one-way clutch locks the stator when the impeller is turning faster than the turbine.

This causes the stator to route oil flow over the impeller vanes properly.

Then, when turbine speed almost equals impeller speed, the stator can freewheel on its shaft so not to obstruct flow.

Therefore, the stator is rotated or locked depending on the direction from which the fluid strikes against the vanes

3.5 THE DRIVE SHAFT OR PROPELLER SHAFTconnects the transmission output shaft to the differential pinion shaft.

3.5.1 DRIVE LINE ASSEMBLYThe drive line assembly has several important functions. It must perform the following: Send turning power from the transmission to the rear axle

assembly. Flex and allow up-and-down movement of the rear axle

assembly. Provide a sliding action to adjust for changes in drive line

length. Provide a smooth power transfer. The assembly provides a path through which power is

transmitted from the transmission to the drive axle assemblies or auxiliary equipment.

3.5.1.1. The drive line assembly consists of the following:

Slip yoke: - connects the transmission output shaft to the front universal joint.

Front universal joint: - the swivel connection that fastens the slip yoke to the drive shaft.

Drive shaft: - a hollow metal tube that transfers turning power from the front universal joint to the rear universal joint.

Rear universal joint: - a flex joint that connects the drive shaft to the differential yoke.

Rear yoke: - holds the rear universal joint and transfers torque to the gears in the rear axle assembly.

3.5.2 THE UNIVERSAL JOINT (U-JOINT):- is used to connect the drive shaft to the transmission output shaft and the differential pinion gear shaft. And it is composed of three fundamental units consisting of a journal (cross) and two yokes.

3.5.3 CENTER SUPPORT BEARINGS Vehicles, having a long wheelbase, are equipped with a

drive shaft that extends from the transmission or transfer case to a center support bearing and a drive shaft that extends from the center support bearing to the rear axle.

The center support bearing bolts to the frame or underbody of the vehicle. It supports the center of the drive shaft where the two shafts come together.

A sealed ball bearing allows the drive shaft to spin freely.

The outside of the ball bearing is held by a thick, rubber, doughnut-shaped mount.

The rubber mount prevents vibration and noise from transferring into the operator's compartment.

CENTER SUPPORT BEARINGS

3.6 DIFFERENTIAL The automotive component which is commonly known

as the differential consists of two parts, the final gears and the differential gears, and has the following functions:

1. Final reduction2. Differentiation3. Directional change of drive power (front engine, rear

wheel drive models)Limited-Slip Differential (LSD) One of the wheels tends to slip and thus fails to transmit driving

torque when the vehicle is running g on a muddy road or making a sharp turn. The LSD overrides the normal differential function so that a larger portion of the drive torque is supplied to the wheel with the better road grip.

Differential

Final gears The final gears of the differential consist of the drive pinion and the ring gear. It is a helical gear on front-wheel-drive vehicles while hypoid bevel gear on rear-wheel-drive vehicles.

3.6.1 DIFFERENTIAL GEARS

3.6.2 PRINCIPLE OF THE DIFFERENTIAL GEAR UNIT

3.6.3 BASIC CONSTRUCTION OF THE DIFFERENTIAL GEAR UNIT

3.6.4 BASIC FUNCTION OF THE DIFFERENTIAL GEAR UNIT 3.6.4.1 Straight ahead travel

3.6.4.2 Turning

The end!

Thank you!